Pipeline pig tools are devices that are used in a variety of applications, including inspection, cleaning, coating, and cutting of the pipeline. It is well known to perform in-line inspections of a pipe by magnetic flux leakage ("MFL") technology. With this inspection practice, an in-line pipe inspection tool is propelled through the pipeline by the product flowing therein, which for example may be oil or gas. The vehicle is propelled along the pipeline by the fluid or gas flow reacting with resilient cups that are mounted around the body of the vehicle and that are in contact with the pipe internal wall. The fluid or gas flow provides the necessary driving force to propel the vehicle by a differential pressure acting across the resilient cups.
As the tool passes through the pipeline, a strong magnetic field is induced into the pipe wall by an inspection tool attached to the vehicle. Defects in the form of discontinuities will cause redistribution of the magnetic flux around the defect. This results in some of the lines of magnetic flux leaking out into the surrounding medium. Though there is a constant magnetic flux leakage, a defect will cause a deviation in the flux leakage field which can be detected. The inspection tool may embody an electromagnet which is battery powered or permanent magnets to induce the magnetic flux field into the pipe wall.
Two sets of steel brushes mounted on the vehicle are commonly used in conjunction with the magnet to constitute the magnetic north and south poles of the magnetic flux field. It is necessary to maintain constant contact between the two sets of brushes and the internal surface of the pipe to ensure an uninterrupted magnetic flux field within the pipe wall. A plurality of transducers mounted on the vehicle are used to detect deviations in the magnetic flux leakage field indicating a defect in the pipe wall. The two sets of steel brushes employed with the magnetizer also act to support the inspection tool during its travel through the pipe.
In order to obtain useful and reliable data, slow inspection tools are used that normally travels at speeds less than four meters per second (m/s). As most gas pipelines operate at velocities far in excess of this speed, reducing pipeline flow velocities to provide an optimum MFL measurement environment is one accepted standard for MFL corrosion measurement. However, even at these lower speeds variations in the differential pressure can lead to fluctuations in vehicle speeds, with consequently high and unacceptable accelerations and declarations, resulting in poor MFL data acquisition. Usually the differential variations are caused by shape deformations in the pipeline wall, such as changes in wall friction characteristics, welds, junctions, bends and/or changes in the pipeline wall thickness.
One method of accounting for these changes in differential pressure is to utilize a speed control which features a bypass valve to allow gas within the pipeline to bypass the resilient cups and hence control the differential pressure across the cups. The use of a bypass system has not proved completely satisfactory.
Low MFL tool measurement speed and lack of active speed control bypass capabilities generally resulted in a plethora of economic and operational problems for high pressure gas pipeline operators. Reducing gas or fluid velocities to a fraction of normal throughput velocity was not uncommon and generally resulted in an operational pipeline outage. Economic impact due to lost throughput and operational problems associated with attempting to reduce product flow caused major concerns. Other active speed controlled tools for pipeline have been previously conceived, but it is believed that practical attempts at operating and acquiring MFL data at approximately low tool velocities, without operational impact, have been unsuccessful.
Another concern in MFL inspection tools is excessive wear and deformation of the steel brushes utilized in creating and maintaining the magnetic flux field. After extended use of the inspection tool, these steel brushes tend to wear and deform as a result of the combination of the weight of the inspection tool being supported, the weight of the fluid in the pipe above the inspection tool and contact with the interior surface of the pipe. Changes in wall friction characteristics, welds, junctions, bends and/or changes in the pipeline wall thickness also tend to increase the wear and deformation of the steel brushes. This results in misposition of the inspection tool during travel through the pipe and discontinuous contact between the steel brushes and the interior pipe wall surface. This adversely affects the desired uninterrupted magnetic flux field in the pipe wall necessary for effective defect detection. This condition is further exacerbated when pipe diameters vary.
With these applications the brushes are required to extend sufficiently to support the tool with increased diameter pipe and then compress when inspecting smaller diameter pipe. During extension of the brushes, there must be sufficient force exerted by the brushes against the pipe wall to provide the required contact to ensure an uninterrupted magnetic flux field within the pipe wall. Compression results in excessive wear and deformation of the brushes for in-line inspection tools. Therefore there is a need for a mechanical support device that ensured the best possible consistent sensor/wall engagement to optimize detection of magnetic flux leakage, without operational impact.
Similar problems are encountered in the other fields of pipeline pigging tools where speed control and/or support of the pipeline pig is a necessary feature and/or where excessive wear or deformation of tools contacting the interior surface of the pipeline is a problem. For example, pipeline cleaning pigs commonly comprise a body that support one or more cleaning, scraping or crushing tools for cleaning the interior of the pipeline. Differential variations in the pipeline caused by changes in wall friction characteristics, welds, junctions, bends and/or changes in the pipeline wall thickness often cause excessive wear, deformation or breakage of the tools. If the cleaning pig suffers a catastrophic failure while inside the pipeline, the metal fragments from the tools may become lodged in the wall of the pipeline or in valves or pipeline junctions, or they may damage downstream equipment such as pumps or sensors.
It would be desirable to design a suspension system for use with an in-line pipeline pigging tool that would be operable in average high pressure pipeline systems without providing an operational impact on the pipeline system or product throughput. Such an invention would minimize velocity variations due to changing line conditions by introducing drag into the system that is consistent, tunable and relatively constant through varying wall thickness, be modular in design for ease of serviceability and interchangeability, and durable to withstand potential speed excursions, while minimizing loading due to pipeline constraints commonly found in the industry.